Newton's Third Law

Newton's Third Law - Chapter 7. Newtons Third Law Chapter...

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Chapter 7. Newton’s Third Law Chapter 7. Newton’s Third Law Chapter Goal: To use Newton’s third law to understand interacting objects.
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Ch. 7 – Student Learning Objectives • To learn how two objects interact. • To identify action/reaction pairs of forces. • To understand and use Newton’s third law. • To understand how to use propulsion forces and tension forces.
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OMIT section 7.2 (#1-7) in workbook, but don’t xxxxx it out We’ll do a modification during class.
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Newton’s Third Law
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Action-reaction Pair If object A exerts a force on object B, then object B exerts a force on object A. The pair of forces (due to one interaction), is called an action/reaction pair . The action/reaction pair will never appear in the same free body diagram.
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Tactics: Analyzing interacting objects
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Example - analyzing interacting objects A person pushes a large crate across a rough surface. Identify the objects that are systems of interest Draw free-body diagrams for each system of interest. Identify all action/reaction pairs with a dashed line.
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Forces involved in pushing a crate – FBD of person and crate
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Propulsion Force The force label f p shows that the static friction force on the person is acting as a propulsion force. This is a force that a system with an internal source of energy uses to drive itself forward.
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Propulsion forces
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Freebody Diagrams – Workbook exercises 1-7 Draw a freebody diagram of each object in the interacting system. Show action/reaction pair with red/orange dotted lines. Draw force vectors in another color. Label vectors with standard symbols. Label action/reaction pairs F AonB , F BonA for example.
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A fishing line of negligible mass lifts a fish upward at constant speed. The line and the fish are the system, the fishing pole is part of the environment. What, if anything, is wrong with the free-body diagrams?
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A fishing line of negligible mass lifts a fish upward at constant speed. The line and the fish are the system, the fishing pole is part of the environment. What, if anything, is wrong with the free-body diagrams? The gravitational force and the tension force are incorrectly identified as an action/reaction pair. The correct action reaction pair is…? Action/reaction pairs are never on the same free body diagram. Mass of line considered negligible so no weight force necessary.
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An acceleration constraint is a well- defined relationship between the acceleration of 2 (or more) objects. In the case shown,
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Newton's Third Law - Chapter 7. Newtons Third Law Chapter...

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